By Edison Fong, WB6IQN
The DBJ-1: A VHF-UHF
Dual-Band J-Pole
Searching for an inexpensive, high-performance dual-band base
antenna for VHF and UHF? Build a simple antenna that uses a single
feed line for less than $10.
T
wo-meter antennas are small compared to those for the lower frequency bands, and the availability
of repeaters on this band greatly extends
the range of lightweight low power
handhelds and mobile stations. One of the
most popular VHF and UHF base station
antennas is the J-Pole.
The J-Pole has no ground radials and
it is easy to construct using inexpensive
materials. For its simplicity and small size,
it offers excellent performance. Its radiation pattern is close to that of an “ideal”
dipole because it is end fed; this results
in virtually no disruption to the radiation
pattern by the feed line.
The Conventional J-Pole
I was introduced to the twinlead version of the J-Pole in 1990 by my long-time
friend, Dennis Monticelli, AE6C, and I
was intrigued by its simplicity and high
performance. One can scale this design to
one-third size and also use it on UHF.
With UHF repeaters becoming more popular in metropolitan areas, I accepted the
challenge to incorporate both bands into
one antenna with no degradation in performance. A common feed line would also
eliminate the need for a duplexer. This article describes how to convert the traditional single band ribbon J-Pole design to
dual-band operation. The antenna is enclosed in UV-resistant PVC pipe and can
thus withstand the elements with only the
antenna connector exposed. I have had this
antenna on my roof since 1992 and it has
been problem-free in the San Francisco
fog.
The basic configuration of the ribbon
J-Pole is shown in Figure 1. The dimensions are shown for 2 meters. This design
was also discussed by KD6GLF in QST.1
That antenna presented dual-band resonance, operating well at 2 meters but with
a 6-7 dB deficit in the horizontal plane at
UHF when compared to a dipole. This is
attributable to the antenna operating at its
third harmonic, with multiple out-ofphase currents.
I have tested single-band J-Pole configurations constructed from copper pipe,
450 Ω ladder line, and aluminum rod.
While all the designs performed well, each
had shortcomings. The copper pipe J-Pole
matching section would be exposed to the
1
J. Reynante, KD6GLF, “An Easy Dual-Band
VHF/UHF Antenna,” QST , Sep 1994, pp 6162.
Figure 2––This
EZNEC plot shows
the difference in the
radiation patterns
between a vertical
half-wave radiator
operated at its
fundamental
(146 MHz) and third
harmonic (445 MHz).
At the fundamental,
most of the energy
is directed at right
angles to the
antenna—and to
distant receivers.
Figure 1––Basic diagram and dimensions
for the original 2-meter ribbon J-Pole.
From February 2003 QST © ARRL
to a low feed point impedance suitable for
coax feed. This is done with a ¼ wave
matching stub, shorted at one end and
connected to the ½ wave radiator’s high
impedance at its other end. Between the
shorted and high impedance ends there is
a point that is close to 50 Ω. This is where
the feed line is attached.
Creating the Dual-Band DBJ-1
Figure 3––The 2 meter J-Pole modified for
both VHF and UHF operation. These
measurements are approximate (see text).
air, raising a durability question. The aluminum design would be faced with a similar issue in the salt air of the San Francisco
Bay area. I favor the use of 300 Ω twin
lead because it is easily obtainable and
inexpensive. An advantage of the copper
pipe design was an 8 MHz bandwidth––
about twice that exhibited by the twin lead
version. That was expected, since the copper pipe had a much larger diameter than
the twin lead elements used in that version. My final decision was to be based on
aesthetics, cost and durability...but the antenna had to be a true dual-band design.
How the J-Pole Works
The basic J-Pole antenna is a half-wave
vertical radiator, much like a dipole. What
separates this design from a vertical dipole
is the method of feeding the half-wave
element. In a conventional dipole or
groundplane, the radiation pattern can be
disrupted by the feed line and there is usually a tower or some other support that acts
as a reflector as it is frequently parallel to
the antenna. The J-Pole pattern resembles
that of an ideal vertical dipole because of
its minimal interaction with the feed line.
The performance of this J-Pole is, theoretically at least, equal to a ¼ wave radiator over an ideal ground.
The J-Pole also matches the high
impedance at the end of a ½ wave radiator
So how can one add UHF to the conventional 2-meter J-Pole? First of all, a
half-wave 2 meter antenna does resonate
at UHF. Resonating is one thing, but working well is another. The DBJ-1 not only
resonates, but also performs as a ½ wave
radiator on both bands. An interesting fact
to note is that ½ wave center-fed dipoletype antennas will resonate at odd harmonics (3rd, 5th, 7th, etc). This is why a 40
meter center-fed ½ wave dipole can be
used on 15 meters. Similarly, a 150 MHz
antenna can be used at 450 MHz. However, the performance of the antenna at the
third harmonic is poor when it is used in a
vertical configuration. At UHF (450 MHz)
the ½ wave radiator becomes 3/ 2 wavelengths long. Unfortunately, at UHF, the
middle ½ wavelength is out of phase with
the top and bottom segments and the resulting partial cancellation results in
approximately 2 dB less gain in the horizontal plane compared to a J-Pole operating at its fundamental frequency.
Maximum radiation is also directed away
from the horizon. Thus, although the
J-Pole can be made to work at its third
harmonic, its performance is poor, often
6-8 dB below that of a groundplane.
Figure 2 shows a polar plot of a vertical
½ wave radiator operating at its fundamental (146 MHz) and third harmonic
(445 MHz) frequencies. Note the difference in energies of the two frequencies.
What is needed is a method to decouple
the extra length of the 2 meter radiator at
UHF in order to create independent ½
wavelength radiators at both VHF and
UHF. The DBJ-1 accomplishes this by
using a coaxial stub, as shown in the antenna drawing of Figure 3.
There is 18 inches of RG-174 transmission line connecting the bottom RF connector to the radiating element. Eighteen
inches was chosen so that the bottom portion of the antenna housing can be used to
mount the antenna without disturbing its
electrical characteristics. [The use of
RG-174 coax in this design limits the
power the antenna can handle to less than
60 W at low SWR. By substituting
RG-213, RG-8 or RG-58 cable, power ratings can be improved considerably. However, the length of the decoupling stub at
the UHF antenna may have to be recalculated, because of the change in velocity
factor (VF) of the different cable.—Ed.]
The 16½ inch matching stub of 300 Ω
twin lead works like a ¼ wave stub at VHF
and a ¾ wave stub at UHF with virtually
no penalty, except for a slight 0.1 db loss
from the extra ½ wavelength of feedline.
By experimentation, the 50 Ω point was
found to be 1¼ inches from the shorted
end of the stub. Although the impedance
at this point is slightly inductive, it is still
an excellent match to 50 Ω, with an SWR
of approximately 1.3:1.
Connected to the open end of the
matching stub, the radiating element for
UHF is 11¼ inches long. The stub and
radiator are constructed of a single piece
of twin lead, separated from the matching
stub by a ¼ inch notch in one conductor,
as shown in Figure 3. The extra wire in
the twin lead radiator sections radiates
along with the driven wire, creating a thick
element that is shorter than its free-space
equivalent. To terminate the UHF radiating section, a shorted stub, using RG-174
coaxial cable, is used. As with the input
matching stub, the open end presents a
high impedance and is connected to the
upper end of the UHF radiating section.
Note that the stub is only an open-circuit
at UHF, acting as a small inductance instead, at VHF.
The RG-174 stub connects to the upper section of 300 Ω twin lead and that
completes the VHF radiating element.
Note that the total length of the UHF and
VHF radiating elements plus the coaxial
stub do not add up to a full ½ wavelength
at VHF because the inductance of the coaxial stub acts to shorten the antenna
slightly.
Construction Details
The dimensions given in Figure 3
should be considered a starting point for
adjustment, with final tuning requiring an
SWR analyzer or bridge. During the
antenna’s construction phase, I started at
the feed point (see Figure 3) and after each
section was assembled, the input SWR
was checked. After the ¼ wave VHF
matching section is connected to the 11¼
inch UHF ½ wave section, check the SWR
at UHF. Then add the ¼ wave UHF shorted
RG-174 stub. The stub will require trimming for minimum SWR at UHF. Start
with the stub 10–15% long and trim the
open end for lowest SWR. As a last step,
add the 17 inch section of twin lead.
Again, this section should be trimmed for
the lowest SWR at your frequency of
choice in the 2 meter band.
To weatherize the antenna, enclose it
in ¾ inch schedule-200 PVC pipe with end
caps. These can be obtained from your local hardware or building supply store.
When sliding the antenna into the PVC
tubing, I found no need to anchor the antenna once it was inside. [If larger coaxial
From February 2003 QST © ARRL
Table 1
Measured Relative Performance of the Dual-Band Antenna at 146 MHz
Received Signal Strength
Difference from Reference
VHF ¼ Wave Mobile
Reference
–24.7 dBm
0 dB
VHF Flex Antenna
(“Rubber Duck”)
–30.5 dBm
–5.8 dB
Table 2
Measured Relative Performance of the Dual-Band Antenna at 445 MHz
VHF ¼ Wave Mobile
VHF Flex Antenna
Reference
(“Rubber Duck”)
Received Signal Strength
Difference from Reference
–38.8 dBm
0 dB
–45.3 dBm
–6.5 dB
Standard VHF J-Pole
DBJ-1 J-Pole
–24.3 dBm
+0.4 dB
–23.5 dBm
+1.2 dB
Standard VHF J-Pole
DBJ-1 J-Pole
–45 dBm
–6.2 dB
–38.8 dBm
0 dB
Figure 4––The
Advantest R3361
spectrum analyzer
used in the test.
cable is used for the stub, it is likely that
the top of the antenna will require some
glue or foam to hold the antenna in place
because of the additional cable weight.
—Ed.] The 300 Ω twin lead is sufficiently
rigid so as not to bend once it is inside the
pipe. Install an SO-239 connector in the
bottom end cap. Once the antenna is
trimmed to the desired operating frequency, glue both end caps and seal around
the SO-239 connector. Presto! For a few
dollars, you’ll have a dynamite antenna
that should last for years.
The antenna should be supported only
by the lower 12 inches of the housing to
avoid interaction between the matching
stub and any nearby metal, such as an antenna or tower. The results from the antenna are excellent considering its
simplicity.
Measured Results
Brian Woodson, KE6SVX, helped me
make measurements in a large parking lot,
approximating a fairly good antenna
range, using the Advantest R3361C spectrum analyzer shown in Figure 4.
The transmitter was a Yaesu FT-5200
located about 50 yards from the analyzer.
The reference antenna consisted of mobile
From February 2003 QST © ARRL
¼ wave Motorola ground plane antennas
mounted on an NMO connector on the top
of my vehicle. The flex antenna (“rubber
duck”) was mounted at the end of 3 feet
of coax held at the same elevation as the
groundplane without radials. The J-Pole
measurements were made with no
groundplane and the base held at the same
height as the mobile ground plane.
Table 1 gives performance measurements
at 146 MHz, while Table 2 gives those
same measurements at 445 MHz.
As can be seen in the UHF results, the
DBJ-1 outperforms the standard 2 meter
J-Pole by about 6 dB (when used at UHF),
a significant difference. The standard 2
meter J-Pole performance is equivalent to
a flex antenna at UHF. Also note that there
is no significant difference in performance
at 2 meters between the DBJ-1 and a standard J-Pole. The flex antenna is about
6 dB below the ¼ wave mobile antenna at
both VHF and UHF. This agrees well with
the previous literature.
The completed antenna can be seen
mounted to the author’s roof in Figure 5.
If you do not have the equipment to
construct or tune this antenna at both VHF
and UHF, the completed antenna is available from the author, tuned to your desired
Figure 5––The completed antenna
mounted to the roof.
frequency. The cost is $20. E-mail him for
details.
Ed Fong was first licensed in 1968 as
WN6IQN. His Extra Class came with
WB6IQN. He obtained the BSEE and MSEE
degrees from the University of California at
Berkeley and his PhD from the University of
San Francisco. A Senior Member of the IEEE,
he has seven patents and two-dozen published
papers in the area of communications and integrated circuit design. Presently, he is employed by the University of California at Berkeley teaching graduate classes in RF design
and is a Senior Member of the Technical Staff
at Foveon Corporation in Santa Clara,
California. You can contact the author at
1163 Quince Ave, Sunnyvale, CA 94087;
edison_fong@hotmail.com.
FEEDBACK
In “The DBJ-1: A VHF-UHF Dual-Band
J-Pole” [Feb 2003, p 40], replace “VHF”
with “UHF” in the headings of Table 2, columns 1 and 2. Column 3 remains “VHF,”
as it refers to the use of a 2 meter VHF
J-Pole on its third harmonic. Also, the area
immediately to the left of the RG-174 stub
should not be shaded. The decoupling stub
is in series with two separate pieces of twinlead.
In “The DBJ-1: A VHF-UHF Dual-Band
J-Pole” (Feb 2003, pp 38-40), the length of
the RG-174 matching stub should be shortened a bit to get the antenna closer to a 1:1
SWR. Using the formula for line length versus frequency and wavelength,
L = (VF×984×N) / f
where
L = length (in feet),
VF = velocity factor,
N = number of wavelengths and
f = frequency in MHz